FIG. 1 is a view showing a circuit configuration of a discharge lamp lighting apparatus according to a related art. In the discharge lamp lighting apparatus, between a DC power source Vcc and a common potential (for example, the ground), there is a switching circuit 1 in which four switching elements Q1 to Q4 are bridge-connected. The switching elements Q2 and Q4 are n-channel MOS transistors and the switching elements Q1 and Q3 are p-channel MOS transistors. An output of the bridge-connected switching circuit 1 is connected through a resonant capacitor C1 to a primary winding P1 of a first transformer T1, and also, is connected through a resonant capacitor C2 to a primary winding P2 of a second transformer T2.
A first end of a secondary winding S1 of the first transformer T1 is connected to a first electrode of a cold cathode fluorescent lamp (hereinafter simply referred to as a discharge lamp) 2 and a second end of the secondary winding S1 is connected through a current detection resistor R1 to the common potential. A second electrode of the discharge lamp 2 is connected to a first end of a secondary winding S2 of the second transformer T2. A second end of the secondary winding S2 of the second transformer T2 is connected through a current detection resistor R2 to the common potential.
A current passing through the secondary winding S1 of the first transformer T1 is detected as a voltage generated at the resistor R1 and a current passing through the secondary winding S2 of the second transformer T2 is detected as a voltage generated at the resistor R2 (to be explained later in detail). The voltage generated by the resistor R1 is transferred through a diode D1 to an error amplifier 3 and the voltage generated by the resistor R2 is transferred through a diode D3 to the error amplifier 3.
The error amplifier 3 compares the voltage transferred from the resistor R1 through the diode D1 or the voltage transferred from the resistor R2 through the diode D3 with an internally generated reference voltage and sends a resultant error voltage to a PWM comparator 5. The PWM comparator 5 compares a triangular wave generated by a triangular wave generator 4 with the error voltage sent from the error amplifier 3 and generates a pulse signal whose pulse width corresponds to the error voltage. The pulse signal has a wide pulse width if the error voltage is large and a narrow pulse width if the error voltage is small. The pulse signal generated by the PWM comparator 5 is sent to a frequency divider 6.
The frequency divider 6 divides the frequency of the pulse signal sent from the PWM comparator 5, to generate two drive signals for each pulse and sends the drive signals to first and second drivers 7 and 8, respectively. The first driver 7 provides the switching element Q1 with the output of the frequency divider 6 as a drive signal and the switching element Q2 with a phase-inverted signal of the output of the frequency divider 6 as a drive signal. The second driver 8 provides the switching element Q3 with the output of the frequency divider 6 as a drive signal and the switching element Q4 with a phase-inverted signal of the output of the frequency divider 6 as a drive signal.
As a result, a period in which the switching elements Q1 and Q4 simultaneously turn on and a period in which the switching elements Q2 and Q3 simultaneously turn on are determined according to the voltages detected by the resistors R1 and R2. The switching elements Q1 and Q2 or the switching elements Q3 and Q4 never simultaneously turn on. The period in which the switching elements Q1 and Q4 simultaneously turn and the period in which the switching elements Q2 and Q3 simultaneously turn on are alternately produced.
Operation of the discharge lamp lighting apparatus of the related art having the above-mentioned configuration will be explained. When the switching elements Q1 and Q4 turn on, a current supplied from the DC power source Vcc passes through a route extending along the switching element Q1, the capacitor C1, the primary winding P1 of the first transformer T1, the switching element Q4, and the common potential line, to apply voltage to the capacitor C1 and the primary winding P1 of the first transformer T1. As a result, the capacitor C1 and an inductance of the primary winding P1 of the first transformer T1 resonate to produce a current having a sinusoidal waveform.
Also, when the switching elements Q1 and Q4 turn on, the current supplied from the DC power source Vcc passes through a route extending along the switching element Q1, the capacitor C2, the primary winding P2 of the second transformer T2, the switching element Q4, and the common potential line, to apply voltage to the capacitor C2 and the primary winding P2 of the second transformer T2. As a result, the capacitor C2 and an inductance of the primary winding P2 of the second transformer T2 resonate to produce a current having a sinusoidal waveform.
The secondary winding S1 of the first transformer T1 and the secondary winding S2 of the second transformer T2 are wound to generate high voltages sufficient to light the discharge lamp 2. The secondary winding S1 of the first transformer T1 and the secondary winding S2 of the second transformer T2, therefore, generate sinusoidal high voltages Vout1 and Vout2, respectively, that have opposite phases. Due to this, on the secondary side, a current passes in a direction A through a path Ip′ extending along the secondary winding S1 of the first transformer T1, the discharge lamp 2, the secondary winding S2 of the second transformer T2, the resistor R2, the resistor R1, and the secondary winding S1 of the first transformer T1, to light the discharge lamp 2. At this time, the resistor R2 generates a voltage proportional to the current passing through the discharge lamp 2, and the voltage is transferred through the diode D3 to the error amplifier 3. On the other hand, the resistor R1 generates a voltage that reversely biases the diode D1 so that the diode D1 turns off to output no voltage.
When the switching elements Q2 and Q3 turn on, a current from the DC power source Vcc passes through a path extending along the switching element Q3, the primary winding P1 of the first transformer T1, the capacitor C1, the switching element Q2, and the common potential line, to oppositely apply voltage to the capacitor C1 and the primary winding P1 of the first transformer T1. As a result, the secondary winding S1 of the first transformer T1 generates a sinusoidal high voltage of opposite phase.
Also, the current from the DC power source Vcc passes through a path extending along the switching element Q3, the primary winding P2 of the second transformer T2, the capacitor C2, the switching element Q2, and the common potential line, to normally apply voltage to the capacitor C2 and the primary winding P2 of the second transformer T2. As a result, the secondary winding S2 of the second transformer T2 generates a sinusoidal high voltage of normal phase.
On the secondary side, a current passes in a direction B through the path Ip′ extending along the secondary winding S2 of the second transformer T2, the discharge lamp 2, the secondary winding S1 of the first transformer T1, the resistor R1, the resistor R2, and the secondary winding S2 of the second transformer T2, to light the discharge lamp 2. At this time, the resistor R1 generates a voltage proportional to the current passing through the discharge lamp 2, and the voltage is transferred through the diode D1 to the error amplifier 3.
On the other hand, the resistor R2 generates a voltage to reversely bias the diode D3 so that the diode D3 turns off to output no voltage. As a result, the error amplifier 3 receives a current detection signal formed by alternately combining the voltages generated by the resistors R1 and R2. Based on the current detection signal, the PWM comparator 5 generates a pulse signal to turn on/off the switching elements Q1 to Q4. With this, a current passing through the discharge lamp 2 is controlled to be constant.